Resistencia

4.1. Evolution of the Wadi Dabsa basin

Fieldwork at Wadi Dabsa clarified the preliminary desktop mapping of landscape units using remote sensing data, and provided a solid framework to begin to interpret the artefacts we recorded from the basin within a dynamic landscape context.

The regional landscape mapping had suggested that the basalt flow F4 at the eastern end of the basin may have post-dated formation of the highest elevation tufa, thus potentially providing a minimum age for tufa deposition, and also a potential mechanism by which tufa formation may have been interrupted by blockage by basalt of the pre-existing drainage system. The observation we made of tufa overlying part of the northwestern boundary of this basalt flow, however, shows that tufa deposition post-dated the emplacement of the flow and, in the absence of any further field evidence showing basalt emplacement over tufa, indicates that tufa formation, at least in this part of the basin, occurred more recently than the youngest basalt flow. This sequence, of tufa overlying basalt, is repeated over the entire basin, especially in the incised wadis, and indicates that the tufa was forming in, and filling, a basin whose underlying morphology was controlled by the basalt flows. It is likely, however, that there was more than one phase of tufa deposition in the basin, and that deposition of tufa and basalt may well have overlapped. The absolute dating of the tufa and basalt flows should test this hypothesis.

It seems likely that the input of water and dissolved CaCO3 into the Wadi Dabsa basin has both a surface water and a groundwater component, although the balance between the two requires additional investigation within the region. It is also likely that the CaCO3 is ultimately derived from the limestones and shales of the Asir Mountains, approximately 60 to 80 km to the east of the survey location, and that Ca might also be derived from the weathering of the mafic basalt at, and surrounding, the site. Clarification of carbonate source should be resolvable in future using 87Sr/86Sr isotopic ratios of the carbonate. The possibility of a groundwater component in tufa formation in the Wadi Dabsa basin is supported by the following observations:

(i) It was observed that the east-west flowing wadi to the south of the artefact survey area has no visible tufa deposits, whereas the deeply incised wadi channel between the basin and the downstream fan contains extensive, thick tufa deposits;

(ii) Many of the tributary wadis on the eastern margin of the basin do not contain tufa deposits;

(iii) The higher elevation tufa deposits in the northeast sections of the two sub-basins occur where there is no obvious fluvial channel input. It is possible, however, that the basalt lava flows in these areas have infilled and cut off former fluvial channels, so that drainage is now via shallow groundwater and perhaps even in the unsaturated zone through the basalt flows. Channels may also have incised below the level of the uppermost tufa. Vertical boreholes drilled through the floor of the basin would help to clarify these issues.

4.2 Geomorphological Context of the L0106 & L0130 Artefacts

Observations from the 2017 field season have revealed a complex relationship between the artefacts and tufa formation. The tufa-coated handaxe discovered on the northeastern side of Grid L0130 shows that a period of tufa deposition occurred after manufacture and deposition of the artefact. Yet in the test pits excavated in Grid L0106, artefacts appear to lie within a sediment layer directly overlying the tufa, suggesting that deposition of the artefacts post- dates the formation of the tufa. Such variability in tufa-artefact relationships is, however, unsurprising given the potentially long time span of activity represented by the presence of ESA, MSA and LSA artefacts at the site, and the evidence we have so far for complexity of tufa formation within the basin. The lithic artefact assemblages and the tufa are probably both the product of multiple phases of human activity and tufa deposition across the basin, phases that potentially occurred over extended periods of time.

The stratigraphy of the test pits, and the observation of new artefacts found exposed on the previously collected areas of the L0106 grid, suggest that at least some of the artefacts at L0106 were buried within a silty sand layer that overlies the tufa that was subsequently deflated by wind and water erosion, exposing these artefacts to weathering and erosion at the surface. The origin of this layer is currently unknown Ð it may be a remnant of a soil that formed on the surface of the tufa. Elsewhere in the basin, a greyish silty sand material was observed underlying tufa structures, again highlighting the complexity of tufa formation and its subsequent weathering and erosion.

The presence in 2017 of numerous artefacts on the area of L0106 that underwent full collection in 2015 may be the result of one of two possibilities. Either, they were missed in the original survey, or they have become exposed since that survey. A number, but not all, of

the region which may have intensified deflation of the artefact-containing sediments, primed perhaps by the 2015 team members disturbing a previously stable surface by trampling it during survey. A similar situation was observed by one of us (PCF) at semi-arid field sites in western New South Wales, Australia (Fanning et al., 2009). The increase, however, is of an order of magnitude, and the distribution of artefacts across the landforms units surveyed in 2017 still shows patterning consistent with the rest of the grid, indicating that this variation is not simply an artefact of observing the distributions before and after a two-year hiatus.

Geomorphological mapping and observations of the artefact distributions across the grids has allowed the examination of whether the artefact distribution at the grid scale is influenced by geomorphic processes that control the preservation, exposure and visibility of artefacts. There are low counts of artefacts on the landform units most likely to exhibit surface conditions that inhibit the visibility of artefacts, such as the Drainage Depression unit and, to some extent, the Lower Slope unit, with its mix of fine sediment and large clasts.

The relatively high density of artefacts recorded within the Crest landform unit which has abundant sediment cover and therefore, theoretically, low artefact visibility, is an example of how the interplay between preservation and exposure of artefacts is key to where artefacts are observed. The Crest is the landform unit that preserves the greatest extent of the artefact- bearing sediment layer. The low relief of the landform means that, while there is active deflation occurring across the surface, it is the fine-grained sediments that are being eroded. The larger clasts, including the artefacts, are not moving very far laterally, if at all, but instead form a ÔpavementÕ or ÔlagÕ of clasts on the land surface in which the visibility of artefacts is high. The stepped nature of the ÔUpper SlopeÕ unit mirrors this situation, with the flatter areas behind the bare tufa ridges and barrages exhibiting a similar ÔpavementÕ where sediment is preserved, and containing slightly higher counts of artefacts than the quadrants with a predominance of bare tufa. The tufa ridges, in contrast, are environments in which artefacts on the surface, are susceptible to downslope movement because of their exposure on irregular or sloping surfaces. This mixture in artefact preservation and exposure across the two different types of surface condition within the ÔUpper SlopeÕ landform unit explains why the artefact counts within this landform lie between those of the Lower Slope unit and the Crest unit. Testing of these models in the L0130 grid is not really possible given the rapid survey that was undertaken there in comparison to the full collection carried out in L0106. In addition, the surveyed squares did not intersect the Crest landform unit in this grid.

4.3 The Wadi Dabsa Lithic Assemblage in Techno-Typological Context.

The assemblage from the Wadi Dabsa comprises a mixture of ESA/Acheulean pieces including a collection of bifaces, large cutting tools and cleavers, as well as an extensive range of MSA/Middle Palaeolithic pieces including both an extensive collection of manufacturing debitage as well as retouched pieces. All of these materials appear to be made on locally available volcanic rocks, that can be accessed, at present, both at the site itself and throughout the Wadi Dabsa basin. In a similar fashion to sites examined elsewhere in southwestern Saudi Arabia, the abundance of locally available raw materials would appear to have created a situation in which hominins did not need to maximise the efficiency with which they used materials. As a result, many of the pieces are simply retouched with just the working tips of bifaces, for example, finely retouched to produce rectilinear edges and the buts remaining often either cortical or very simply retouched. There are a few examples of lithic materials that are not obviously occurring in the local basin (including quartz, chert and indurated shale) and these pieces also show greater preparation and retouch. It seems likely at the moment that these pieces represent the discard of later, post MSA hominins. Likewise, there are a few examples of tool types (endscrapers and burins) that are normally associated with Upper Palaeolithic assemblages in Europe, although such tools have been found in MSA assemblages in Africa.

Looking beyond the Wadi Dabsa basin, the lithic assemblage itself contains pieces that are similar to examples found during the course of the fieldwork by the UK-Saudi team since 2013, but the quantity and, specifically, the quality of the technological information present at Wadi Dabsa is more extensive and complete in sequence than at any other site located so far. The variability in evident skill among the examples of prepared cores also suggests that the assemblage represents a cross-section of ages and experience in lithic manufacture; this is unique to this site.